Replacement for Instant Messaging

[dz]

Couldn't figure out which newsgroup to ask this one, but...

A client is disallowing its users access to instant
messaging due to too much outside of office use. So, they
need some type of app that will run internally, only, that
functions like instant messenger. I created a user
interface for net send, but that is very limited in that
it doesn't have a reply feature and totally interrupts the
recipient until pressing the OK button.

Any ideas?

Thanks.